Advanced composite materials are used in the aircraft industry due to their optimum specific mechanical properties when compared with equivalent metallic parts. These properties are a consequence of the high specific properties of the reinforcing fibres (graphite in most cases) and their directionality: an adequate combination of layers with different fibre orientations allows a high optimisation of the mechanical properties of the resultant laminate.
However, this directionality is extensive to the thermo-mechanical behaviour of composite materials, as reinforcing fibres and resins commonly used as matrix (epoxy in most cases) have very dissimilar expansion coefficients, circumstance that promotes a very anisotropic expansion coefficient, which is very low, even negative in the direction of the reinforcement (−2.10−6 to 3.10−6° C.−1), and in the range of 3.10−5 to 6.10−5° C.−1 in the transversal direction.
This thermo-mechanical anisotropy complicates the optimisation of flat laminates (and forces to use symmetric and equilibrated ply stacking sequences), curved laminates (this case is specially sensible, and usually requires a complex tailoring of the tooling), and monolithic stiffened structures.
In the last case, an optimum structural optimisation of a manufactured part of an aircraft would required the use of very different stacking sequences for the skin and stiffeners of an aircraft, situation that inexorably drives to highly stressed or distorted structures. A modification in the stacking sequences can reduce this effect, but with an undesirable weight penalty.
This effect can be theoretically predicted, and thermal induced distortions can be reduced or corrected with an adequate design of the tooling (the shape of the tooling must be corrected in order to compensate the predicted deformation of the part to be manufactured). However, this method is always expensive (in most cases it complicates the manufacture of the tooling), very inaccurate, as deformation depends on many variables (with a high risk of scraps as a consequence), and non flexible (a slight modification in the part makes the tooling unusable).
A process for manufacturing composite material primary structures with curing female tooling has been disclosed in EP-A-1 136 236. Also, EP-A-1 231 046 discloses a method for manufacturing elements of composite materials by the co-bonding technique.